For several reasons poly(dimethylsiloxane) (PDMS) is one of the most commonly used materials in microfluidic chip fabrication. Compared to silicon and glass devices, PDMS based chips can be manufactured much faster, easier and cheaper by means of soft lithography. Due to its elasticity pumps and valves can be introduced into PDMS devices. Furthermore, PDMS can be cured at low temperature, it is transparent down to 280 nm, biologically inert and non-toxic as well as permeable to gases. It also readily seals with other materials, such as glass and poly(methyl methacrylate), which allows for the fabrication of hybrid chips. However, significant limitations concerning the application of PDMS in microfluidic devices arise from the high hydrophobicity of the material. For instance, the creation of oil-in-water emulsions inside microfluidic chips requires an effective wetting of the microchannel walls with the continuous aqueous phase. Therefore, a surface modification is often necessary, although rather challenging because of the inertness of PDMS.
In literature numerous ways of PDMS surface modification can be found. One possible approach comprises the exposure of PDMS to various energy sources, such as oxygen plasma. In this context, the generation of hydrophilic surfaces by oxidation is only temporary though since PDMS is known to regain its original hydrophobic surface properties over time, a phenomenon referred to as hydrophobic recovery.
Alternatively, chemical vapor deposition (CVD) can be used to introduce permanent coatings and hence adjust the surface properties of PDMS. However, since this method requires unhindered access of the vapor to the substrate it is limited to the modification of non-assembled microfluidic chips. This is a significant drawback as the coating must then be stable enough to endure the bonding procedure typically involving plasma treatment.
Furthermore, PDMS surfaces can be modified covalently, most commonly via graft photo-polymerization. A simple one-step strategy is available which allows for the tailoring of PDMS surface properties by grafting various monomers. Again problems arise for assembled channels though, since polymerization predominantly occurs in the channel lumen rather than at the walls. The pre-adsorption of a suitable photo-initiator solves this problem, but requires additional preparation steps making the procedure more elaborate. Similarly, other covalent modification strategies, such as the generation of a glass coating via sol-gel methods offer the possibility to modify PDMS permanently, but again in a rather complex, labor-intensive and time-consuming manner.
Another surface modification method is based on the layer-by-layer (LbL) self-assembly of polyelectrolyte multilayers (PEMs) by alternate adsorption of polycations and polyanions. This versatile approach was introduced by the group of Decher [G. Decher, J. D. Hong, J. Schmitt, Thin Solid Films, 1992, 210/211, 831-835; G. Decher, Y. Lvov, J. Schmitt, Thin Solid Films, 1994, 244, 772-777; G. Decher, Science, 1997, 277, 1232-1237].
Based on these results Katayama et al [H. Katayama, Y. Ishihama, N. Asakawa, Anal. Chem., 1998, 70, 2254-2260; H. Katayama, Y. Ishihama, N. Asakawa, Anal. Chem., 1998, 70, 5272-5277] developed an LbL procedure allowing for the treatment of capillary inner walls. Various other groups like Barker et al, [S. L. R. Barker, M. J. Tarlov, H. Canavan, J. J. Hickman, L. E. Locascio, Anal. Chem., 2000, 72, 4899-4903; S. L. R. Barker, D. Ross, M. J. Tarlov, M. Gaitan, L. E. Locascio, Anal. Chem., 2000, 72, 5925-5929.] Henry et al [Y. Liu, J. C. Fanguy, J. M. Bledsoe, C. S. Henry, Anal. Chem., 2000, 72, 5939-5944] and Hahn et al. [K. W. Ro, W-J. Chang, H. Kim, Y.-M. Koo, J. H. Hahn, Electrophoresis, 2003, 24, 3253-3259] are using similar protocols in order to modify surface properties of microfluidic channel walls. However, all of these coating procedures rely on the manual flushing of a microfluidic chip with different polyelectrolyte solutions. As additional washing steps need to be carried out in between and all polyelectrolyte and washing solutions have to be individually injected into the device, kept inside the channels for some time and removed afterwards this method can easily become a tedious and labor-intensive task, too. Weitz et al. (A. R. Abate, D. A. Weitz, Small 2009, 5, 2030) have used photolithographic techniques to realize alternating wettability profiles inside microchannels for producing multiple emulsions.
Further background prior art can be found in U.S. Pat. No. 6,860,980, US2002/0053514, US2004/0084312, U.S. Pat. No. 6,402,918, and WO2005/052035.
There is therefore a need for improved techniques for surface modification of channels of a microfluidic device.